Recent work in our laboratory on the content of transfer RNA (tRNA) in rabbit reticulocytes and the roles of the different tRNA species in hemoglobin synthesis indicates that these molecules are specialized in their abundance for hemoglobin synthesis and that the availability of some tRNA species may limit the rate and amount of hemoglobin synthesis in red blood cell precursors (Smith, D.W.E., Science 190, 529 (1975)). The work proposed in this application includes the determination of the kinds and amounts of tRNA attached to reticulocyte polysomes, ribosomal monomers, and ribosomal subunits. The results will reveal the amount of ribosomal tRNA of the different species that is actively participating in hemoglobin synthesis and will provide evidence concerning whether the rate of peptide bond formation on nascent globin chains is uniform or whether it varies. Studies will be done on the specific sites in hemoglobin incorporated by the tRNA isoaccepting species for arginine, isoleucine, leucine, and perhaps other amino acids, and the utilization of tRNA isoaccepting species in hemoglobin synthesis will be studied by determining the extent of their attachment to ribosomes or to polysomes. The disappearance of tRNA from reticulocytes as they mature into erythrocytes will be examined, and the half life of tRNA for different amino acids and the liability of this part of the reticulocyte protein biosynthetic machinery will be determined. The possibility that hemoglobin synthesis ceases in the maturing reticulocyte because of loss of certain tRNA species will be considered. The tRNA content of basophilic erythroblasts, early precursors in red cell differentiation, will be compared with that of reticulocytes to determine if it is specialized for hemoglobin synthesis at this early stage. Since tRNA may well be a determinant of the amount of hemoglobin in erythrocytes, an understanding of the content and use of these molecules in red cell precursors may reveal mechanisms for deficient hemoglobin synthesis that occurs in some of the hemoglobinopathies and thalassemias.